If you are considering staying at a place where it might be good to have a reservation (e.g., commercial campground on a weekend) you can always call a few days ahead to check. They may tell you that they will find a place for you even if all the sites are full. Back in '99 our group of 13 managed to cross the U.S. with only once not being able to camp where we had intended (Presque Isle S.P. in Erie, PA) because of capacity problems, and we stayed in a lot of commercial campgrounds.